Thin film photovoltaics (PV) based on Cu(InxGa1-x)Se2 (CIGS) and similar technologies such as Cu2ZnSnS4 (CZTS) or Cu2ZnSnSe4 (CZTSe) are promising candidates for low-cost, high-efficiency solar cell applications. CIGS technology has demonstrated the highest energy conversion efficiency among all polycrystalline thin film PV technologies.
Many materials used in thin film technologies are deposited according to a multi-stage method, in which certain elements are introduced alternatively into a deposition matrix. For example, with respect to a CIGS film, a three stage method has been demonstrated to be effective, with In and Ga deposited in a first stage, Cu deposited in a second stage and In and Ga deposited in a third state. Deposition during each of the foregoing three stages occurs in the presence of excess Se. Substantial detail concerning the fabrication of CIGS and related thin films according to a multi-stage method may be found in commonly owned U.S. Pat. No. 5,441,897 titled “Method of Fabricating High-efficiency Cu(In,Ga)(Se,S)2 Thin Films for Solar Cells” which patent is incorporated herein by reference for all matters included therein.
It is common that the optimum ratio of constituent elements in a high-performance thin film PV device is off-stoichiometry. Furthermore, the target compositional ratios for acceptable thin film properties and device performance can be relatively narrow. For example, the best performing Cu(InxGa1-x)Se2 solar cells fabricated according to the three stage method noted above have a Cu/(In+Ga) ratio in the general range of 0.8-0.9. Cell performance is perhaps optimized when the Cu/(In+Ga) ratio is close to 0.9. If the Cu/(In+Ga) ratio significantly exceeds 0.9 however, device performance will drop sharply with increasing Cu/(In+Ga) ratio.
Furthermore, the distribution of In+Ga between the first and third stages and the total film thickness are also important parameters that significantly affect the final solar cell performance. Precise control of the elemental composition, elemental distribution between stages and film thickness is difficult in a multiple stage process for many reasons, including but not limited to the fact that deposition rates of different stages may vary, even for stages depositing the same elements. Deposition rate variations can be intentional, for example as needed for optimizing certain material properties or unintentional, for example as a result of system instability. A need exists for methods and apparatus providing for the monitoring and control and ideally the real-time monitoring and control of multi-stage thin film deposition processes. In particular, a need exists for the monitoring and control and ideally the real-time monitoring and control of one or more of the following parameters: (1) the elemental composition of a deposited thin film; (2) the distribution of elements among stages; and/or (3) the total film thickness.
The embodiments disclosed herein are intended to overcome one or more of the limitations described above. The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.